I was told that the No's 4 +5 windows were fitted to the origanal kc135 to allow the Pilot's to see the lower fuselage markings/lights on the tanker aircraft during flight refueling. As the forward fuselage on the Kc-135, 707, 720, 727 and 737 is basicaly the same, they were left in.
On a personal note they are a pain in the ar*e to change and should have been removed a long time ago!
Rgds Dr.I.